Extension Limit of a Straight-Swirling Mixed Jet Bit and Its Influential Factors in Radial Jet Drilling

Abstract

Radial jet drilling (RJD) is applied to low-permeability and unconventional natural gas. A self-propelled bit chiefly affects the borehole length and drilling efficiency. Herein, for a straight-swirling mixed jet (SSMJ) bit, a prediction model of the drilling extension limit $L_{\text{EL}}$ was established by analysing the forces and pressure loss of RJD system. $L_{\text{EL}}$ of a specific project was calculated and the pressure loss and forces distribution were obtained. Additionally, the influence laws of the main factors on $L_{\text{EL}}$ were studied using the established model. The results indicated that the high-pressure hose contributes the main system pressure loss, and the recoil force of backward nozzles is the sole driving force of RJD. The recoil force of forward nozzle and friction between the hose and borehole constitute the main resistances. Increasing the pump pressure $p_{\text{b}}$ , backward nozzle diameter $d_{\text{b}}$ , and jet diffusion angle ${\theta }_{\text{x}}$ and reducing forward nozzle diameter $d_{\text{f}}$ , impeller central hole diameter $d_{\text{m}}$ , and backward nozzle inclination angle ${\theta }_{\text{b}}$ are conducive to improving $L_{\text{EL}}$ . However, larger $p_{\text{b}}$ and $d_{\text{b}}$ increase the hydraulic consumption, besides larger ${\theta }_{\text{x}}$ or smaller $d_{\text{f}}$ , and $d_{\text{m}}$ will reduce the rock-breaking capability. From a sensitivity analysis, $p_{\text{b}}$ and $d_{\text{m}}$ have the maximum and minimum influence on $L_{\text{EL}}$ , respectively. Finally, prediction value $L_{\text{EL}}$ of an oil well in Eastern Sichuan can be up to 63.7 m. The results provide a guidance for the hydraulic parameters and key component selection, additionally bit structure optimization of RJD.